The present invention relates to proximity sensors. More particularly, the present invention relates to a system and method for compensating for the effects of a cable used with a proximity sensor.
U.S. Pat. No. 5,180,978 of Postma et al. issued Jan. 19, 1993 and assigned to the assignee of the present invention (hereinafter the ""978 patent), describes a two wire proximity sensor having a sensing coil movable into proximity with a metal member or target and provides for the direct measurement of the AC and DC resistances of the coil and, in one embodiment the imaginary impedance component (i.e., the reactance). A microprocessor utilizes these values and provides an output indicative of the distance between the coil and the target. The measurement of the distance to the target affects the difference between the AC resistance and the DC resistance and this difference does not vary much with temperature. Since the wire has equal values of AC resistance and DC resistance, the effect of the wire is cancelled by taking the difference of these two resistances.
One difficulty has been encountered in the use of the invention of the ""978 patent and that occurs when the lengths of the wires to and from the sensor become long enough that the capacitance introduced thereby causes phase shifts which prevent the measurement of the AC resistance of the sensor. Although using low frequency can minimize this effect, attempts to make this effect extremely small motivate the use of a frequency so low that the eddy currents in the metal target no longer produce the desired change in the AC resistance. Aircraft manufacturer requirements have been changed from requiring plus or minus 1.0 mm accuracy to requiring an accuracy of plus or minus 0.1 mm and to requiring that the measurement be insensitive to temperature between 77xc2x0 C. to +125xc2x0 C. with cable lengths of 3 m to 80 m in an unshielded twisted pair.
A preferred embodiment of the present invention overcomes some of the problems associated with the new accuracy requirements and allows great accuracy over extreme cable length variations with minimal problems with noise, sensor temperature variations, and capacitance. The basic change over the prior systems is to measure the admittance of a proximity sensor and an interposed transmission cable. Assumptions about sensor characteristics and cable characteristics are made, and an admittance corresponding to the assumed characteristics is derived. The assumptions are adjusted to obtain a correspondence between the measured admittance and the derived admittance. When the assumptions have been updated a specified number of times or when a desired accuracy threshold has been met, the adjusted assumptions may be used to determine sensor characteristics, such as the separation between the proximity sensor and the target object.